There is an increasing need for microlasers, particularly of the type having single longitudinal modes.
Various techniques are known for limiting the output of a microlaser to essentially a single longitudinal mode. The number of modes at which a laser can oscillate is generally determined by the number of the modes that are established in the laser cavity whose gain is sufficient to initiate the onset of oscillations. Accordingly, the number of possible oscillation modes can be kept small by using a short optical cavity for the laser.
However, for output powers in excess of about one milliwatt, it is difficult to achieve reliable single longitudinal mode operation with monolithic-rod solid state lasers. Pump limited single mode operation has been obtained from a monolithic out-of-plane ring (MISER) but at the expense of considerable complexity in crystal fabrication and housing design. Alternatively, single mode pulses have also been obtained by gain switching a monolithic rod, a technique which also tends to be complex.
The use of etalon mode selectors have been proposed as an alternative to the approaches described above and they offer promise for a simple rugged single mode laser. In microlasers, the gain medium, or active lasing element, has been used as a mode selecting transmission etalon within the laser cavity. However, this technique results in lasers which tend not to be particularly stable.
There has also previously been proposed a solid state single mode laser in which an absorptive element, such as a metal film, is located at a null in the electric field of the standing wave in the optical cavity of a desired single axial mode, to little affect such mode but to attenuate other axial modes whose electric field is not zero at the region of the metal film. In particular, a paper which appeared in the Bell System Technical Journal, May-June 1969 describes at pages 1416-1418 a laser system in which a Nd-doped YAG rod, 3 centimeters long, is side-walled pumped in an elliptical cylinder, and the optical cavity comprises a partially transmissive curved mirror that serves as the first reflector of the cavity and a fused silica flat having a thin metal coating on one surface and a high reflectivity coating on the other surface for serving as the second reflector of the cavity. The two reflectors were spaced apart 20 centimeters, nearly seven times the length of the YAG rod, which results in a laser longer than desirable for many applications.
Subsequently in a paper in the IEEE Journal of Quantum Electronics, August 1971, pages 381-387 there is analyzed for use as a mode selector for a single mode YAG laser, typically at least fifteen centimeters long, an etalon comprising a two millimeter quartz flat with a thin nichrome metal film on one surface and a number of quarter-wavelength dielectric layers on the opposite surface to provide a partially transmissive mirror for use as an output coupler with the amount of output coupling being determined by the number of layers. This mode selector too depends on locating the thin metal film at a null in the electric field of the standing wave pattern of a desired single axial mode. The paper additionally analyzes, also for use as a mode selector an etalon comprising a quartz flat each of whose surfaces is coated with a thin metal film and concludes that the latter etalon has properties for mode selection superior to the former etalon. The paper finally proposes and describes tests on a sidewall-pumped Nd-doped YAG laser in which the first described multilayer reflector-absorbing film etalon is combined with an intra-cavity tilted Fabry-Perot etalon to form a two-component mode filter that is described as overcoming the difficulties of the other arrangements at higher levels of output power.
In Optics Letters, Volume 10, No. 2, February 1985, there is described a miniature YAG laser which is end-pumped with a laser diode. The YAG rod is provided with one curved end surface that is suitably coated to serve as a reflective mirror at the laser frequency but to remain transparent to the pumping light that is focused by a gradient index optical fiber lens into this end of the rod. The other end of the rod was also curved but suitably coated to be highly reflective at the pumping frequency and partially transmissive at the laser frequency for output coupling. However, the maximum continuous output power that was reported to have been reached was 4.4 milliwatts with an overall electric-to optic efficiency of 1.5%, although it has proven difficult to reproduce these results. The laser was found to oscillate both in a single axial and in a single transverse mode, apparently without use of any separate frequency mode selector, with rods three and five millimeters long. Temperature tuning was used.
The various prior art discussed above indicates that there is still need for a single mode microlaser which can provide tens of milliwatts of output power with a simple rugged structure and in one aspect the invention is directed to this end. In particular, in an illustrative embodiment the invention is a microlaser with an optical cavity less than one centimeter long. In an alternative embodiment, by use of an etalon that includes a birefringent flat, there results a microlaser whose output includes two longitudinal modes whose polarizations are orthogonal to one another and whose frequencies are sufficiently different to be readily separated.